Aortoenteric fistulas: spectrum of CT findings
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- Raman, S.P., Kamaya, A., Federle, M. et al. Abdom Imaging (2013) 38: 367. doi:10.1007/s00261-012-9873-7
This article reviews the causes of aortoenteric fistulas, diagnostic options, and important CT findings.
Aortoenteric fistula, a rare but potentially fatal entity, presents a significant challenge to radiologists in diagnosis, largely because of its subtle and nonspecific imaging findings. These fistulas can be divided into primary and secondary forms, depending on the presence or absence of prior aortic reconstructive surgery, but the secondary form is more common. Typical CT findings, which can overlap with those seen in perigraft infection, aortitis, infected/mycotic aneurysms, perianeurysmal fibrosis, and the immediate post-operative period after placement of a graft, include: Effacement of the fat planes around the aorta, perigraft fluid/soft tissue thickening, ectopic gas, tethering of adjacent thickened bowel loops towards the aortic graft, and in rare cases, extravasation of contrast from the aorta into the involved segment of bowel.
KeywordsComputed tomographyAortoenteric fistulaPerigraft infectionAortitis
A rare (seven cases of primary aortoenteric fistulas/100 million), life-threatening condition, aortoenteric fistulas present a significant diagnostic challenge [1, 2]. Early diagnosis is critical, as mortality in the absence of intervention is virtually 100% . While endoscopy and other imaging modalities do have a role, the diagnosis of a fistula is critically dependent upon CT, which has a number of suggestive, albeit subtle, findings .
Due to the rarity of the disorder, there are fewer than 300 reported cases in the literature, and very few systematic descriptions of the key CT findings . In this pictorial essay, we present 10 patients with aortoenteric fistulas, each of whom demonstrate varying suggestive findings on CT. As these cases demonstrate, the CT findings can be quite difficult to discern without a high index of suspicion.
Clinical presentation and etiology
Aortoenteric fistulas can be divided into primary and secondary forms: Primary fistulas, considered rare, occur in a native aorta without a history of prior intervention. While typically caused by an atherosclerotic penetrating ulcer, other causes can include diverticulitis, foreign bodies, aortitis, appendicitis, and gastrointestinal malignancies [5–8].
Secondary fistulas are seen in the setting of prior surgery or intervention, and are more common, with a reported incidence as high as 0.6% in patients with previous aortic surgery or graft placement [3, 9]. Patients at particular risk include those who undergo emergent surgery for a ruptured aneurysm, have post-operative complications such as reoperation or bowel injury, and those with endoleaks or stent migration . The cause of secondary fistulas is thought to be the result of chronic perigraft infection or prolonged pressure upon the bowel by a graft .
While fistulas can occur with any part of the gastrointestinal tract, the classic location is the transverse portion of the duodenum, which is involved in 60% of cases. Less common locations include the remainder of the duodenum, jejunum and ileum, stomach, sigmoid colon, and ascending/descending colon, each of which is involved in less than 5% of cases .
The nonspecific nature of these findings leads to considerable overlap with a number of other disorders. Of these, perigraft infection is the most important, as it can look identical to a fistula. The clinical history (especially gastrointestinal bleeding or hematemesis) should raise diagnostic suspicion for a fistula, and while ectopic gas can be seen in both disorders, it is more common with the presence of a fistula. Secondly, the CT features of a fistula can be normal findings in the immediate post-operative period, and ectopic gas (normal up to 1 month) and perigraft fluid (normal up to 3 months) should not be misinterpreted as a fistula. Other potentially similar appearing entities include aortitis, mycotic aneurysms, and perianeurysmal fibrosis, all of which can demonstrate periaortic inflammation, fluid, or soft tissue .
Ancillary diagnostic findings
Endoscopy can exclude other causes of significant upper GI bleeding, although it is not uncommon for an incidental ulcer to be mistaken as the primary cause of bleeding [2, 12]. Moreover, the fistula itself is rarely seen by endoscopy, and the diagnosis is often only hinted at by the presence of blood in the duodenum. Only 25–62.5% of aortoenteric fistulas could be diagnosed using endoscopy in one series .
While many assume that conventional angiography is the gold-standard in diagnosis, angiographic diagnosis is actually quite difficult, as most of these cases have slow or intermittent flow into the fistula (Fig. 5) . Nuclear medicine Technetium-labeled red blood cell (RBC) scans are also limited, with poor spatial resolution and a lack of specificity .
Classically, the treatment for aortoenteric fistulas involved resection of the infected graft, bowel resection, and creation of an extra-anatomic bypass graft [6, 10]. Unfortunately, mortality rates are extremely high (up to 90%), and there is a high rate of limb amputations (up to 50%) and aortic stump disruptions (up to 38%) [10, 14]. Over the last decade, the trend has been towards the use of endovascular techniques for both primary and secondary fistulas. The use of endovascular techniques eliminates the most common fatal complication of open surgery—dehiscence of the aortic stump, and also has a lower risk of perioperative complications [10, 14].
As the cases we have presented demonstrate, the CT features of aortoenteric fistulas can be quite subtle. While the CT findings are often not completely specific, the burden of diagnosis still falls upon CT, as other modalities such as endoscopy, nuclear medicine studies, and angiography have significant limitations. Ultimately, a high index of suspicion is required given the potentially fatal consequences of a missed diagnosis.